The use of resonant circuits integrally formed as part of security tags and labels for theft prevention of retail goods is well known. Such security tags have been in commercial use for many years. These tag circuits are tuned to a given radio frequency, such that a detectable resonant condition will result. Should shoppers pass through the transmitting and receiving units at the portals of the store with these tags still attached to the merchandise, an alarm will sound.
It is current operating practice to remove these tags at the check-out counter, such that legitimate sales will proceed without sounding the alarm when the shopper exists the store.
It is also common practice to paste-over these circuits at the point of sale with a metallized sticker. These metallized paste-overs in effect prevent the resonant circuit from causing an alarm, thus allowing honest shoppers to pass safely through the exist.
While the aforementioned techniques have worked well in the marketplace, they nonetheless present certain drawbacks to the security system.
First, the need to remove these tags or to perform a paste-over, requires additional employee time and vigilance. The operating cost factor is increased as a result.
Secondly, this procedural step alerts would-be shoplifters to ways to defeat the detection system. Once cognizant of the process, shoplifters can themselves paste-over the security tags or labels, or remove them prior to leaving the store.
More recently, it has been suggested that the resonant tags be unobtrusively deactivated by electronic means. One such technique causes a "shorting" or "arcing" across the tuned circuit capacitor by means of an induced current, supplied by a radio frequency signal of higher energy than the detecting signal. This type of deactivating system is disclosed in U.S. Pat. No. 4,567,473, issued to George J. Lichtblau on Jan. 28, 1986.
Another electronic deactivation technique is disclosed in U.S. Pat. No. 3,967,161, issued to George J. Lichtblau on June 29, 1976. In this patent, a resonant circuit is shown having a fusible link, formed of a narrowed, or necked-down portion of the conductor that constitutes the inductor portion of the resonant circuit. This portion is caused to open by the relatively high intensity, induced current of the deactivating radio frequency signal.
While the aforesaid techniques perform the task of electronic deactivation in an adequate manner, they are not without certain weaknesses.
The fusible link technique requires a deactivating current which is so intense that the radio frequency signal needed to induce it can cause problems in conforming to the regulations and requirements of the Federal Communications Commission.
As for the capacitor shorting technique, this may not always provide a clean and definable short circuit across the resonant circuit, thus leaving the deactivation and disablement of the security tag in doubt.